Some boats easily seen sailing (2,3)
I believe the answer is:
at sea
'sailing' is the definition.
(synonyms)
'some boats easily seen' is the wordplay.
'some' indicates a hidden word (some parts of the word are used).
'AT SEA' is hidden within 'boats easily seen'.
(Other definitions for at sea that I've seen before include "The men in the dock are not" , "Confused, uncertain" , "Muddled on the ocean?" , "Confused (in the main?)" , "Bewildered on the ocean?" .)